Electrostatic printing process

ABSTRACT

The use of crystalline zeolitic molecular sieves in electrically conductive backing paper provides improved results in a conventional electrostatic printing process. The molecular sieves are employed in the backing paper in an amount sufficient to impart an electrical resistivity of less than 1 X 1012 ohm-cm over the range of relative humidity of from 5 to 90%.

United States Patent 1191 Breck May 20, 1975 [54] ELECTROSTATIC PRINTINGPROCESS 3,033,233 ISQeIph et a].i ll7/2())l6 U);

,6 t l. [75] Inventor: Donald W. Breck, White Plams, awyer e a FOREIGNPATENTS OR APPLICATIONS 1 Assigneer Union Carbide Corporation, New1,092,600 11/1967 United Kingdom 96/1.5 York, N.Y. 449,713 5/1969 Japan96/1.5 [22] Filed: Nov. 13, 1969 21 App] 371 285 PrimaryExaminer-Michael F. Esposito Rel t d U S A r f D t Attorney, Agent, orFirm-Richard G. Miller ae pp1ca10n aa [62] Division of Ser. No. 527,065,Feb. 14, 1966,.

abandoned, which is a division of Ser. No. 166,948, Jan. 17, 1962,abandoned. [57] ABSTRACT [52] U S Cl 96/1 96/1 7/17 The use ofcrystalline zeolitic molecular sieves in elec- 6; trically conductivebacking paper provides improved [51] Int Cl Dzlh 3/66, 603g 7/00 resultsin a conventional electrostatic printing process. [58] Field 7/201 17 5The molecular sieves are employed in the backing 96/1 5 l paper in anamount sufficient to impart an electrical resistivity of less than 1 X10 ohm-cm over the range [56] References Cited of relative humidity offrom 5 to 90%.

UNITED STATES PATENTS 10/1942 Carlson 117/201 UX 2 Claims, N0 DrawingsELECTROSTATIC PRINTING PROCESS RELATED APPLICATIONS This application isa division of application Ser. No. 527,065 filed Feb. 14, 1966, nowabandoned, which in turn is a division of application Ser. No. 166,948,filed Jan. 17, 1962, now abandoned.

This invention relates to an electrostatic printing process whichutilizes a zeolite-containing electrically conductive paper.

There presently exists a need for a paper which is electricallyconductive over a wide range of humidity conditions. Such papers areespecially needed in electrostatic printing processes, computers, andthe like where it is not unusual to operate at relative humidities ofless than 20%. Normally, paper has a specific resistance of l X 10ohm-cm or greater, whereas, a paper having a resistivity of l X 10 to lX 1'0 ohm-cm is desired in electrostatic printing processes for example.

Heretofore, the commercially available conductive papers have beenextremely sensitive to change of humidity. Under conditions of highhumidity, the paper will conduct electricity but as the humidity isdecreased the paper loses its conductivity. As a result, in printingprocesses, for example, there is created the problem of maintaining gooddefinition ofimages to be reproduced under low humidity conditions.

Accordingly, it is the main object of this invention to provide anelectrostatic printing process using an electrically conductive paperwhich will retain its electrical characteristics regardless of thehumidity conditions.

The invention provides an electrostatic printing process utilizing anelectrically conductive paper which will maintain its electricalconductivity over the range of humidity of from about to about 90%.

Whenever the electrical resistivity of a particular specimen is given,it is to be understood that the conductivity of such specimen would bethe reciprocal of the resistivity.

The above objects are accomplished in general by a paper provided with azeolite material, either natural or synthetic, sufficient to impart anelectrical resistivity of less than 1 X 10 ohm-cm over the range ofrelative humidity of from about 5 to 90%.

The process for making such an electrically conductive paper maycomprise either incorporating the zeolite in the paper or coating thepaper with a zeolite to impart to such paper an electrical resistivityof less than 1 X 10 ohm-cm over the range of relative humidity of fromabout 5% to about 90%.

For purposes of this disclosure, impregnating means actually adding thezeolite material into the raw materials to be formed into the paper sothat such zeolite is a basic constituent of the paper.

By coating is meant adding the zeolite to a surface of a product paper.

While the invention well described in further detail hereinbelow withreference to the greatly preferred crystalline zeolite materials such aszeolites A and X it is to be understood that the amorphous or jell typemetal alumino-silicates made synthetically for water softeners and whichhave been called zeolites in the prior art are intended to beencompassed within the metes and bounds of this invention.

The crystalline zeolites as well as the amorphousmetal-alumino-silicates suitable for use in this invention arecharacterized by oxygen tetrahedra containing a silicon or aluminum ion.The aluminum ion with one less positive charge than silicon can onlysatisfy three negative charges of the four oxygens which surround it. Toproduce a stable structure it must have help from another positivelycharged ion. This is the function of the exchangeable ions such assodium and calcium. Further, zeolites suitable for practicing thepresent invention have a framework of silicon-oxygen and aluminumoxygentetrahedra containing passageways therein extending in three directions.

The impregnating of zeolites into the paper to render it electricallyconductive according to the invention can be effected at any point inthe paper making operation or in a later separate operation. Thus, thezeolite may be added, for example, to the water slurries containing thefibrous materials to be formed into paper or the zeolite can be added ata subsequent point in the processing of the slurry, such as the beatingor refining step. The zeolite can also be coated or sprayed onto aproduct paper. The coating of zeolites onto a paper can be accomplishedby any of the methods presently employed for applying inorganic papercoatings. One such method is known as conversion coating and involvesthe application of a coating to paper already in roll form. Anothermethod known as on-machine coating, involves the application of acoating to one or both sides of paper as it is being made, dried andpassed through the paper-making machine. The latter practice isexemplified by such methods as roll-coating, intagliooffset coating,centrifugal-spray coating, and doctorbar coating. Depending on the typeof coating apparatus used, the coating may'be applied to wet paper ordry paper as well as to'paper of intermediate moisture content.

The amount of crystalline zeolite necessary to render a paperelectrically conductive, according to the invention, depends on severalparameters such as paper making techniques, type of paper, particle sizeof zeolite, etc. It has been found that when crystalline zeolitematerial having a particle size of l-2 microns is coated on a paper suchcoating may comprise as low as 1% by weight of the paper stock. However,about 5% by weight is preferred. It is postulated that the lower limitis dependent on obtaining good point to point contact betweencrystalline particles which is possible with particles of the sizedescribed.

When the crystalline zeolite is actually made a constituent of the papersuch point to point contact is not obtained and more zeolite isrequired. In this case the recommended amount of zeolite is from 5% toabout 30% by weight of the paper stock. The maximum amount that can beutilized in either case (coating or impregnating) is limited by the factthat too much zeolite might affect paper strength or crease properties.It has been found that 50% by weight is the practical upper limit.

In the following discussion, the values given are for the purpose ofindicating the general order of magnitude of resistivity that can beachieved by practicing the teachings of the invention, and not for thepurpose of defining the ultimate values or composition attainable bysuch invention. Further the discussion in most part will be directed toan electrically conductive paper for use in reproduction processes andin particular to those based upon an electrostatic principle. Anexemplary electrostatic printing process is described in U.S. Pat. No.2,297,691, issued Oct. 6, 1942, to C. F. Carlson, the disclosure ofwhich is incorporated herein to the extent pertinent. In this process aplane conductive backing is coated with a uniform layer ofphotoconductive insulating material. A strong electrostatic charge isdeveloped on the surface of the insulating material which is thenexposed to light. As a result the illuminated areas become conductiveand drain off a substantial portion of the charge to the conductivebacking leaving a latent electrostatic image on the insulating materialsurface. This latent image is then developed by contacting a dust withthe surface to form an electrostatic dust image on the area of thesurface which remains electrically charged, and then transferring thedust image to a sheet of paper.

In its broad aspects the invention is predicated on the discovery thatwhen a paper is impregnated, incorporated, or coated with a crystallinezeolite, such paper will exhibit electrical characteristics,particularly electrical conductivity, which can be maintainedsubstantially unchanged over varied relative humidity conditions.

A complete discussion of the naturally occurring crystalline zeolitematerials such as chabazite, mordenite, erionite, analcite, faujasite isnot believed to be necessary as there is adequate discussion of suchzeolites in the literature.

Among the synthetic crystalline three-dimensional zeolites admirablysuited for use in the invention are crystalline zeolites A and X whichare described respectively in US. Pat. No. 2,882,243 and US. Pat. No.2,882,244 issued Apr. 14, 1959, to R. M. Milton. Other suitablecrystalline zeolites include those described as follows:

Zeolite F in U.S. Pat. No. 2,996,358

Zeolite Q in US. Pat. No. 2,991,151

Zeolite E in US. Pat. No. 2,962,355

Zeolite T in US. Pat. No. 2,958,952

The term zeolite, in general, refers to a group of naturally occurringand synthetic hydrated metal alumino-silicates, many of which arecrystalline in structure. There are, however, significant differencesbetween the various synthetic and natural materials in chemicalcomposition, physical properties and crystal structure, the latter asevidenced by X-ray powder diffraction patterns.

The structure of crystalline zeolitic molecular sieves may be describedas an open three-dimensional framework of SiO, and A tetrahedra. Thetetrahedra are cross-linked by the sharing of oxygen atoms, so that theratio of oxygen atoms to the total of the aluminum and silicon atoms isequal to two, or O/(Al Si) =2. The negative electrovalence of tetrahedracontaining aluminum is balanced by the inclusion within the crystal ofcations, for example, alkali metal and alkaline earth metal ions such assodium, potassium, calcium and magnesium ions. One cation may beexchanged for another by ion-exchange techniques.

The crystalline zeolites may be activated by driving off at least aportion of the water of hydration. The space remaining in the crystalsafter activation is available for adsorption or adsorbate moleculeshaving a size, shape and energy which permits entry of the adsorbateinto the pores of the molecular sieves.

The crystalline zeolites occur as agglomerates of fine crystals or aresynthesized as fine powders and are preferably tableted or pelletizedfor large scale adsorption uses. Pelletizing methods are known which arevery satisfactory because the sorptive character of the zeolite, bothwith regard to selectivity and capacity, remains essentially unchanged.

Crystalline zeolites in a hydrated state are uniquely suited for thisinvention, since these materials are electrical conductors andfurthermore retain electrical conductivity over a wide range oftemperatures and humidity. It is to be noticed that the invention is notpredicated on the adsorbing properties of the Zeolites but rather on theelectrical properties.

In the first experiments which led to the invention two electrodes aboutone-eighth inch in diameter were inserted about 1 cm. apart into sodiumzeolite A (4A) powder and the electrical resistance measured. Theresistance value was found to be approximately I X 10 for zeolite Type4A. Several other materials were tested in the same way includingcertain materials commonly used as paper coatings or fillers orpigments. These were kaolin clay, a natural calcium silicate known aswollastonite, a natural magnesium silicate known as talc, a naturalamorphous silica known as diatomite, and a finely divided crystallinesilica in the form of a silica flour. None of these materials exhibitedany measurable electrical conductivity under the conditions describedabove.

After the above tests, a piece of paper was coated with zeolite 4Apowder. A water slurry of zeolite 4A powder was prepared and applied toa 3 inch circle of filter paper by a filtration technique. The filterpaper used was essentially a pure cellulose paper. After drying atapproximately C., the ability of the coated paper to dissipate anelectrostatic charge was tested. An electrostatic charge was applied toa gold leaf electroscope. The electroscope was obtained with the coatedpaper and the charge on the scope was dissipated as rapidly as whencontacted with a metal conductor. In contrast, an uncoated paperexhibited no electrical conducting effect.

In subsequent tests, experimental papers were made by incorporating orcoating paper with the following materials:

The three crystalline zeolites were compared with the amorphous sodiumaluminosilicate Zeolex to determine the effect of crystallinity.

The crystalline zeolites were compared with other crystalline ionicmaterials such as sodium chloride and calcium hydroxide, and withkaolin.

Two methods were employed in the preparation of the experimental paperspecimens: (a) the materials listed above were applied in the form of acoating on a surface of a paper; (b) the paper was reconstituted withthe material incorporated within its fibers. The paper chosen for thiswork, was ashless filter paper. This paper was chosen for the followingreasons: (1) the paper contains no original contaminating filler orsizing, and (2) because it consists of loosely packed cellulose fibersthat could be fairly easily reconstituted. Any paper could be so treatedby suitable processes known in the papermaking art.

The coated papers were prepared by dispersing the insoluble powders(crystalline zeoliteskaolin, and the like) in distilled water. Thedispersion was then applied to a circular piece of filter paper, 9 cm.in diameter. The water-soluble sodiumchlor ide and partially solublecalcium hydroxide were dissolved to form a saturated water solution.Then a calculated amount of the water soluble material was added to thesolution and the mixture filtered through the paper specimen. Bysupporting the paper specimen on the glass frit mounted in a pressurefilter a smooth uniform coating was obtained. Each paper specimenprepared in this manner contained about 17 wt-% of the solids. Thecoated paper specimens were all dried in the air at 95C.

In order to prepare paper specimens containing the incorporated filler,the following procedure was used. About 1.53 grams of the filter paperwas dispersed in distilled water in the form of a pulp slurry byagitation in 200 cc. of water in a mixer. The insoluble powders (zeoliteor clays) were then added to the pulp slurry, throughly mixed in ablender and then filtered rapidly onto a matt in a pressure filter ofl-liter capacity under 60 psig. pressure of gaseous nitrogen. The slurrywas later separated from the matt as a paper. The filtering procedureprovided dense paper compacts with a reasonably uniform thickness anddensity. The soluble materials [NaCl and Ca(Ol-l) were prepared in theform of a concentrated solution prior to addition to the water-pulpslurry followed by filtration. The quantity of salt contained in thesaturated solution adsorbed by the paper was considered in computing thecomposition of the final paper compact. Two series of reconstitutedpaper compacts were prepared containing, respectively, l7 wt-% and 70wt-% of the materials described above. In another series, paperspecimens containing varying amounts of zeolite Type 4A powder wereprepared using the procedure above described.

After preparation, the paper specimen was cut into a strip 2 /2 by 1inches. Each end of the strip was fitted to an electrode consisting oftwo rectangular copper plates under spring tension, each measuring 1.0 X0.25

inches. On this basis, the conducting length of each specimen of paperwas 2 inches and the thickness of all of the coated papers was assumedto be constant at tance of all of the other specimens were measuredunder an applied voltage of 500 volts. The time for The resistivity ofall the specimens containing 17 wt-% of the filler was measured at 27C.in a dry nitrogen atmosphere, and in air, at 65% relative humidity. Theinstrumentused in measuring the resistance, of

each paper specimen was the Ty pe H D C bridge, ;distributed by theInstrument Division of Federal Tel'e- 1 phone and Radio Company ofClifton, N]. This instrueach measurement was held constant at 3 minutesso that any possible capacity effect might be kept constant. Resistivitydata for these conditions is tabulated in Table l. Fromthe data in TableI, it is evident that for these samples, a concentration of 17 wt-% ofthe crystalline zeolite reduces the electrical resistivity significantlyat high humidities. It is to be understood that this figure is not thelower limit but merely indicative of the results that can be obtained byincorporating or coating crystalline zeolites on paper.

The resistivity of the paper containing the abovementioned materialsincorporated to the extent of wt-% was measured in air at 5%, 25% and65% relative humidity, at 27C. Prior to equilibration in the humiditychamber, these specimens were dried at C. in air. Resistivity data isgiven in Table II.

The resistivity of the series of paper specimens containing incorporatedcrystalline zeolite Type 4A in a range of varying concentrations wasmeasured in air at 25% and 65% relative humidity at 27C. These data areshown in Table III.

At a relative humidity of 5% at 27C., it is found that the paperspecimens containing 70 wt-% of the crystalline zeolite 13X inparticular, reduced the resistivity of the paper by at least at factorof l X 10 to l X 10", whereas the amorphous material Zeolex was muchless effective and sodium chloride exhibited no effect at all. Theobserved resistance of both the paper control specimen and the papercontaining sodium chloride was greater than 1 X 10 ohm-cm. It isapparent that the resistivity of the paper specimen is dependent uponrelative humidity to varying degrees depending upon the added pigment ormaterial present. The resistivity of the paper with no additives is mostdependent upon relative humidity, since it experiences a resistivitychange of about 1 X10 The paper specimens containing the zeolitematerials are the least dependent upon humidity and exhibit a change inresistivity of about 10. Under the same conditions, the paper specimenscontaining sodium chloride exhibited a change in resistivity of about 1X 10 From the above discussed data, it will be apparent that thecrystalline zeolite materials can be used to achieve an electricallyconductive paper at almost all conditions of humidity. The specificquantity of a crystalline zeolite which must be impregnated in or coated3 on a paper to impart the electrical properties will depend upon themethod'of producing the paper, the'type paper and use of such paper. Aconsiderable reduction in the quantity of crystalline zeolite needed torender the paper electrically conductive can possibly be obtained withmore refined paper making techniques which will be obvious to theartisan in the papermaking art.

While the present invention has been described in reference to twospecific techniques for preparing a -paper containing a crystallinezeolite material so as to render such paper conductive, improvements inthese techniques and other techniques will suggest themselves to thoseskilled in the paper-making art. For example, it is possible to add thereactant materials for making a synthetic zeolite into the slurry andthen permitting the crystals to grow as the raw materials are formedinto a paper.

For example, the particle size of the molecular sieve and the ability todisperse the agglomerate particles What is claimed is:

1. In an electrostatic printing process the improvement which comprisesapplying a uniform layer of photoconductive insulating material to anelectrically conductive paper backing provided with a sufficient amountof crystalline zeolite material to impart to such paper an electricalresistivity of less than I X 10 ohmcm over the range of relativehumidity of from 5 to TABLE I THE RESISTIVITY OF PAPER CONTAINING 17.2WT-% FILLER Specific Resistance (ohm-cm) Dry N2 Atmosphere 65% Relative27C. Humidity, 27C.

Filler Material Coated during application either on the paper in theform of a coating or within the fibers of the paper if so incorporated,are all important in the practice of this invention. The examples andexperimental results discussed above are for the purposes ofillustration only and show that 5 the electrical conductivity of papercan be drastically improved by the application of a molecular sievematerial of the crystalline zeolite type.

90%; developing a strong electrostatic charge on the surface of saidlayer; exposing the charged layer to light source to render theilluminated areas thereof sufficiently conductive to drain off asubstantial portion of said charge to said electrically conductive paperbacken contacting a fine dust with the surface whereby to form anelectrostatic dust deposit on the areas of said surface remainingcharged after exposure and removing excess dust not electrostaticallyheld on 2. A process according to claim 1 wherein the electricallyconductive paper is provided with from about 1% by weight to about 50%by weight of the paper base stock of crystalline zeolite material.

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1. IN AN ELECTROSATIC PRINTING PROCESS THE IMPROVEMENT WHICH COMPRISESAPPLYING A UNIFORM LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL TO ANELECTRICALLY CONDUCTIVE PAPER BACKING PROVIDED WITH A SUFFICIENT AMOUNTOF CRYSTALLINE ZEOLITE MATERIAL TO IMPART TO SUCH PAPER AN ELECTRICALRESISTIVITY OF LESS THAN 1 X 1012 OHM-CM OVER THE RANGE OF RELATIVEHUMIDITY OF FROM 5 TO 90%, DEVELOPING A STRONG ELECTROSTATIC CHARGE ONTHE SURFACE OF SAID LAYER, EXPOSING THE CHARGED LAYER TO LIGHT SOURCE TORENDER THE ILLUMINATED AREAS THEREOF SUFFICIENTLY CONDUCTIVE TO DRAINOFF A SUBSTANTIAL PORTION OF SAID CHARGE TO SAID ELECTRICALLY CONDUCTIVEPAPER BACKING, THEN CONTACTING S FINE DUST WITH THE SURFACE WHEREBY TOFORM AN ELECTROSTATIC DUST DEPOSIT ON THE AREAS OF SAID SURFACEREMAINING CHARGED AFTER EXPOSURE AND REMOVING EXCESS DUST NOTELECTROSTATICALLY HELD ON SAID SURFACE.
 2. A process according to claim1 wherein the electrically conductive paper is provided with from about1% by weight to about 50% by weight of the paper base stock ofcrystalline zeolite material.